Broad-band-cholesteric liquid-crystal film, process for producing the same, circularly polarizing plate, linearly polarizing element, illiminator, and liquid-crystal display

ABSTRACT

A broad band cholesteric liquid crystal film of the present invention is a cholesteric liquid crystal film obtained by coating a liquid crystal mixture containing a polymerizable mesogen compound (a), a polymerizable chiral agent (b) and a photoisomerizable material (c) on a substrate to ultraviolet polymerize a coat of the liquid crystal mixture, and has a reflection bandwidth of 200 nm or more. A broad band cholesteric liquid crystal film of the present invention has a broad reflection band, is of a thin type and can be manufactured in less of manufacturing steps.

TECHNICAL FIELD

The present invention relates to a broad band cholesteric liquid crystalfilm and a manufacturing method therefor. A broad band cholestericliquid crystal film of the present invention is useful as a circularlypolarizing plate (a reflection polarizer). The present invention relatesto a linearly polarizer, a luminaire and a liquid crystal display usingthe circularly polarizing plate. Moreover, the present invention relatesto a polarizing element system using the circularly polarizing plate anda wide viewing angle magnification liquid crystal display using thepolarizing element system.

BACKGROUND ART

Generally, a liquid crystal display has a structure in which a spacebetween glass plates forming transparent electrodes is filled with aliquid crystal and polarizers are arranged before and after the glassplates. A polarizer used in such a liquid crystal display ismanufactured in a procedure in which iodine or a dichloic dye issubjected to be adsorbed to a polyvinyl alcohol film and the film isstretched in a given direction. The polarizer thus manufactured itselfabsorbs light vibrating in one direction and transmits only lightvibrating in the other direction therethrough to thereby producelinearly polarizing light. Therefore, an efficiency of the polarizercould not exceed 50% theoretically, which works as the greatest factorto reduce an efficiency of a liquid crystal display. As the mattersworse about the absorbed light, if a liquid crystal display is operatedwith an increased output of a light source beyond a level, it results ininconveniences that a polarizer is broken down by heat generation due tothermal conversion of absorbed light or that a display quality isdegraded under thermal influence onto liquid crystal layer in a cell.

A cholesteric liquid crystal having a circularly polarized lightseparating function has a selective reflection characteristic reflectingonly circularly polarized light having a direction thereof coincidingwith a helical rotation direction of the liquid crystal and a wavelengthequal to a helical pitch length of the liquid crystal. With thisselective reflection characteristic used, only a specific circularlypolarizing light of natural light in a given wavelength band istransmission-separated and the other light components are reflected andrecycled, thereby enabling a polarizing film with a high efficiency tobe manufactured. In the context, transmitted circularly polarized lightpasses through a λ/4 plate and thereby converted to linearly polarizinglight, and coincidence of a direction of the linearly polarized lightwith a transmission direction of an absorption polarizer used in aliquid crystal display enables a liquid crystal display with a hightransmittance to be realized. That is, in a case where a cholestericliquid crystal film is combined with a λ/4 plate and the combination isused as a linearly polarizer, the linearly polarizer could achieve abrightness twice as that of a conventional absorption polarizer singlyused, which absorbs 50% of incident light, due to no light losstheoretically.

There has been, however, difficulty in covering all the range of visiblelight, since a selective reflection characteristic of a cholestericliquid crystal is restricted to only a specific wavelength band. Aselective reflection wavelength bandwidth Δλ is expressed by followingformula:Δλ=2λ·(n _(e) −n _(o))/(n _(e) +n _(o))where n_(o): ordinary light refractive index of a cholesteric liquidcrystal molecule, n_(e): extraordinary light refractive index of thecholesteric liquid crystal molecule, and λ: central wavelength inselective reflection.

The selective reflection wavelength bandwidth Δλ depends on a molecularstructure of the cholesteric liquid crystal itself. According to theabove formula, if (n_(e)−n_(o)) is larger, a selective reflectionwavelength bandwidth Δλ can be broader, while (n_(e)−n_(o)) is usually0.3 or less. With this value being larger, other functions as a liquidcrystal (such as alignment characteristic, a liquid crystal temperatureor the like) becomes insufficient, causing its practical use to bedifficult. Therefore, a selective reflection wavelength bandwidth Δλ hasbeen actually on the order of 150 nm at highest. A cholesteric liquidcrystal available in practical aspect has had a selective reflectionwavelength bandwidth Δλ only of the order in the range of 30 to 100 nmin many cases.

A selective reflection central wavelength λ is given by the followingformula:λ=(n _(e) −n _(o))P/2where P: helical pitch length required for one helical turn ofcholesteric liquid crystal.

With a given pitch length, a selective reflection central wavelength λdepends on an average refractive index and a pitch length of a liquidcrystal molecule.

Therefore, in order to cover all the range of visible light, there havebeen adopted methods, in one of which plural layers having respectivedifferent selective reflection central wavelengths are laminated, and inanother of which a pitch length is continuously changed in the thicknessdirection to thereby form a positional distribution of selectivereflection central wavelengths.

For example, there can be exemplified a method in which a pitch lengthis continuously changed in the thickness direction (for example, see apublication of JP-A No. 6-281814, a specification of JP No. 3272668 anda publication of JP-A No. 11-248943). This method is such that when acholesteric liquid crystal composition is ultraviolet exposure-cured,exposure intensities on sides of exposure and light emission aredifferentiated therebetween to alter a polymerization speedtherebetween, which provides a change in compositional ratio of a liquidcrystal composition having a different reaction speed in the thicknessdirection.

The bottom line of this method lies in that exposure intensities onsides of exposure and light emission are greatly different therebetween.Therefore, in many of the examples of the prior art described above,there has been adopted a method in which an ultraviolet absorbent ismixed into a liquid crystal composition so as to cause absorptionthereof in the thickness direction to thereby amplify a difference inexposure dosage according to an optical path length.

In a method disclosed in a publication of JP-A No. 6-281814, in which apitch length is continuously altered, necessities arise for a liquidcrystal thickness required for revealing the function to be on the orderin the range of from 15 to 20 μm, and for more of an expensive liquidcrystal in amount in addition to a problem of precise coating of aliquid crystal layer, which disables cost-up to be avoided. Moreover, anexposure time is necessary to be on the order in the range of from 1 to60 min, which leads to a need for a long manufacturing line with anexposure line length in the range of from 10 to 600 m in order to obtaina line speed of 10 m/min. With a reduced line speed adopted, a linelength can be reduced, while a lower manufacturing speed cannot beavoided.

This is because, as described in the publication of JP-A No. 6-281814, aquick change in pitch is difficult to be realized due to a theoreticalissue in controlling a cholesteric pitch caused by a difference inultraviolet exposure intensity in the thickness direction for a changein pitch length in the thickness direction and by a change incompositional ratio due to material transfer caused by a difference inpolymerization speed accompanying the difference in ultravioletintensity. Since, in the publication of JP-A No. 6-281814, pitch lengthsin the short pitch side and the long pitch side are differenttherebetween by as large as on the order of 100 nm, a compositionalratio is necessary to change to a great extent and in order to realizeit, a further necessity arises for a considerable thickness of liquidcrystal, a very weak ultraviolet illumination and a long exposure time.

Since in a method disclosed in a publication of JP-A No. 11-248943,transfer of a material changing a pitch is better than an examplematerial used in the publication of JP-A No. 6-281814, an exposuredosage of the order of 1 min enables a film to be formed. In this caseas well, a necessary thickness is 15 μm, however.

While, in a specification of JP No. 3272668, a temperature condition ina first exposure is altered from that in a second exposure and a timenecessary for a change in compositional ratio in the thickness directionis separately provided in a dark place, a wait time for materialtransfer due to a change in temperature is necessary to be in the rangeof from 10 to 30 min.

A liquid crystal coat thickness, even in the specification of JP No.3272668 and the publication of JP-A No. 11-248943, is about 15 μm and incomparison of the specification and the publication described above withthe publication of JP-A No. 6-281814 in which the liquid crystal coatthickness is required to be about 20 μm, it is understand that anecessity arises for a larger cholesteric liquid crystal thickness and alonger time for material transfer in order to cover all the range ofvisible light with a change in pitch caused by a change in compositionalratio in the thickness direction of one liquid crystal layer.

In a publication of JP-A No. 9-189811, at least three layers arenecessary in order to cover all the range of visible light, and a longwavelength side is covered for betterment of a viewing anglecharacteristic, and the number of necessary laminated layers increasesto as large as 4 to 5 in a case where a measure is taken against obliqueincident light, which leads to more complexity in manufacture steps andincrease in the number of steps, thereby unavoidably resulting inreduction in production yield.

With combination of such a broad band circularly polarizing plate with aretardation plate, a diffuse light source is enabled to emit collimatedlight. Adoption of such a collimated light source and a diffuse plateenables a construction of a viewing angle magnification system in aliquid crystal display.

For example, as shown in a specification of JP No. 2561483 and apublication of JP-No. 10-321025, by inserting a retardation platecontrolled in a way such that a retardation value in a verticaldirection of incidence and a retardation value in an oblique directionof incidence are specifically different from each other betweenpolarizers, an angular distribution of transmitted light receives arestraint and in this case, if an absorption polarizer is used, lightonly in the vicinity of the front face is transmitted, while peripherallight are all absorbed. By using a circularly polarizing plate (areflection polarizer), light only in the vicinity of the front face istransmitted while peripheral light is all reflected. If such an effectis adopted, emission light of a backlight can be condensed andcollimated without being accompanied by absorption loss.

With combination of such a collimated backlight source and a diffuseplate less of backscattering and occurring no polarization cancellation,a viewing angle magnification system can be constructed. As describedabove, in a conventional method in which multiple liquid crystal layersare laminated, however, (the publication of JP-A No. 9-189811), therehas arisen a problem of increased number of steps due to lamination ofmultiple layers, while in a method as disclosed in the publication ofJP-A No. 6-281814 or the specification of JP No. 3272668 in which aliquid crystal layer is thick, there has occurred a problem of cost-up.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a broad bandcholesteric liquid crystal film having a wide reflection band, of a thintype, and capable of manufacturing itself with a less number ofmanufacturing steps and a manufacturing method therefor.

It is another object of the present invention to provide a circularlypolarizing plate using the broad band cholesteric liquid crystal film,and furthermore, to provide a linearly polarizer, a luminaire and aliquid crystal display using the circularly polarizing plate.

It is still another object of the present invention to provide apolarizing element system using the circularly polarizing plate and toprovide a wide viewing angle magnification liquid crystal display usingthe polarizing element system.

The present inventors have conducted serious studies in order to solvethe problems with resultant findings that the objects can be achievedwith the following broad band cholesteric liquid crystal film and amanufacturing method therefor, leading to completion of the presentinvention. That is, the present invention is as follows:

1. A broad band cholesteric liquid crystal film comprising: acholesteric liquid crystal film obtained by coating a liquid crystalmixture containing a polymerizable mesogen compound (a), a polymerizablechiral agent (b) and a photoisomerizable material (c) on a substrate toultraviolet polymerize thereof, having a reflection bandwidth of 200 nmor more.

2 The broad band cholesteric liquid crystal film according toabove-mentioned 1, wherein a pitch length in the cholesteric liquidcrystal film changes continuously.

3. The broad band cholesteric liquid crystal film according toabove-mentioned 1. or 2., wherein the liquid crystal mixture comprisinga photopolymerization initiator (d).

4. The broad band cholesteric liquid crystal film according to any oneof above-mentioned 1. to 3., wherein the polymerizable mesogen compound(a) has one, or two or more of polymerizable functional groups, thepolymerizable chiral agent (b) has one, or two or more polymerizablefunctional groups.

5. The broad band cholesteric liquid crystal film according to any oneof above-mentioned 1. to 4., wherein the photoisomerizable material (c)is at least one kind selected from the group consisting of stilbene,azobenzene and a derivative thereof.

6. A manufacturing method for the broad band cholesteric liquid crystalfilm according to any one of above-mentioned 1. to 5. comprising stepsof: coating a liquid crystal mixture containing a polymerizable mesogencompound (a), a polymerizable chiral agent (b) and a photoisomerizablematerial (c) on a substrate and ultraviolet polymerizing thereof.

7. A circularly polarizing plate comprising the broad band cholestericliquid crystal film according to any one of above-mentioned 1. to 5.

8. A linearly polarizer comprising the circularly polarizing plateaccording to above-mentioned 7. and a λ/4 plate laminating on thecircularly polarizing plate.

9. The linearly polarizer according to above-mentioned 8., thecircularly polarizing plate, which is the cholesteric liquid crystalfilm, is laminating on the λ/4 plate so that a pitch length in the filmis narrowed toward the λ/4 plate continuously.

10. A linearly polarizer comprising an absorption polarizer adhering tothe linearly polarizer according to above-mentioned 8. or 9. so that atransmission axis direction of the absorption polarizer and atransmission axis of the linearly polarizer are arranged in parallelwith each other.

11. A luminaire comprising the circularly polarizing plate according toabove-mentioned 7. or the linearly polarizer according to any one ofabove-mentioned of 8. to 10. on a front surface side of a surface lightsource having a reflective layer on the back surface side thereof.

12. A liquid crystal display comprising a liquid crystal cell in a lightemitting side of the luminaire according to above-mentioned 11.

13. A polarizing element system comprising: a retardation layer (b)having a front face retardation (in the normal direction) of almost zeroand a retardation of λ/8 or more relative to incident light incoming atan angle of 30° or more inclined from the normal direction is arrangedbetween at least two layers of a reflection polarizer (a) havingrespective selective reflection wavelength bands of polarized lightsuperimposed on each other,

wherein the reflection polarizer (a) is the circularly polarizing plateaccording to above-mentioned 7.

14. The polarizing element system according to above-mentioned 13.,wherein a selective reflection wavelength of the at least two layers ofthe reflection polarizer (a) is superimposed on each other in thewavelength range 550 nm±10 nm.

15. The polarizing element system according to above-mentioned 13. or14., wherein the retardation layer (b) is a layer comprising acholesteric liquid crystal phase having a selective reflectionwavelength band other than the visible light region fixed in planaralignment.

16. The polarizing element system according to above-mentioned 13. or14., wherein the retardation layer (b) is a layer comprising a rod-likeliquid crystal fixed in homeotropic alignment state.

17. The polarizing element system according to above-mentioned 13. or14., wherein the retardation layer (b) is a layer comprising a discoticliquid crystal fixed in nematic phase or columnar phase alignment state.

18. The polarizing element system according to above-mentioned 13. or14., wherein the retardation layer (b) is a layer comprising a biaxiallyorienting polymer film.

19. The polarizing element system according to above-mentioned 13. or14., wherein the retardation layer (b) is a layer comprising aninorganic layered compound with negative uniaxiality fixed in alignmentstate where an optical axis thereof is a normal direction of a surfacethereof.

20. A wide viewing angle liquid crystal display comprising at least:

a backlight system containing a polarizing element system according toany one of above-mentioned 13. to 19. to collimate a light from adiffuse light source;

a liquid cell transmitting collimated light;

a polarizing plate arranged on both sides of the liquid cell; and

a viewing angle magnification film, which diffusing transmitted light,arranged on a viewer side of the liquid cell.

21. The wide viewing angle liquid crystal display according toabove-mentioned 20., wherein a λ/4 plate is arranged on the viewer side(the liquid cell side) of the polarizing element system according to anyone of above-mentioned 13 to 19 so that an axial direction of linearlypolarized light transmitted and a transmission axis direction of apolarizing plate on the lower side (the light source side) of the liquidcrystal display are arranged in parallel with each other.

22. The wide viewing angle liquid crystal display according toabove-mentioned 20. or 21., wherein the viewing angle magnification filmis a diffuse plate substantially having neither backscattering norpolarization cancellation.

23. The wide viewing angle liquid crystal display according to any oneof above-mentioned 20. to 22., wherein an each layer is laminated with atranslucent adhesive or a pressure sensitive adhesive.

Action

A broad band cholesteric liquid crystal film of the present inventiondescribed above is obtained by ultraviolet polymerizing a polymerizableliquid crystal mixture and the liquid crystal mixture contains aphotoisomerizable material (c). With such a photoisomerizable material(c) adopted, it is realized to reduce an ultraviolet illumination timeand form a film with a thin coat thickness.

It was reported that the photoisomerizable materials (c) such asazobenze can reversibly control selective reflection band of acholesteric liquid crystal in a photoisomerization reaction, which wasdescribed in Japanese Liquid Crystal Society symposium papers, pp. 66 to69 (1999). For example, a photoisomerization reaction occurs in a waysuch that by illuminating azobenzene with ultraviolet of a wavelength inthe vicinity of 365 nm, a trans-isomer is converted to a cis-isomer,while by illuminating it with visible light of a wavelength in thevicinity of 440 nm or heating, a cis-isomer is converted to atrans-isomer. That is, it has been reported that when a substrate onwhich a liquid crystal mixture containing a photoisomerizable material(c) is coated is illuminated with ultraviolet, a reflection band of acholesteric liquid crystal shifts.

If such a photoisomerizable material (c) is added to a liquid crystalmixture and the mixture is illuminated with ultraviolet so that anultraviolet illumination dosage is distributed in the thicknessdirection, isomerization from a trans-isomer to a cis-isomer advances inthe ultraviolet illumination side. On the other hand, in the oppositeside from the ultraviolet illumination side, isomerization from atrans-isomer to a cis-isomer is harder to advance. Therefore, revealedis a positional distribution of a change in ratio of trans-isomer andcis-isomer in the thickness direction, which enables manufacture of abroad band cholesteric liquid crystal film having a selective reflectionwavelength bandwidth covering all the region of visible light. A broadband cholesteric liquid crystal film thus obtained works as a broad bandcircularly polarizing plate and not only has an optical property equalto that of the liquid crystal films disclosed in the publication of JP-ANo. 6-281184, the specification of JP No. 3272668 and the specificationsof JP-A Nos. 11-248943 and 9-189811 (hereinafter referred to as knownpatent literatures), but also can decrease a thickness thereof, therebyin addition, enabling low-cost manufacture thereof due to greatreduction in manufacturing steps to be realized.

That is, a broad band cholesteric liquid crystal film of the presentinvention can be formed as a thin layer to thereby enable a use amountof an expensive liquid crystal material to be reduced. Moreover, a totalthickness of the liquid crystal layer can be decreased and the number oflaminating steps can also be decreased. As a result, the number of stepsin manufacture can be decreased, thereby enabling cost reduction owingto increase in line speed to be achieved.

A broad band cholesteric liquid film of the present invention describedabove has a broad bandwidth of selective reflection wavelength, which isas broad as 200 nm or more. The reflection bandwidth is preferably 300nm or more and more preferably 400 nm or more. A reflection bandwidth of200 nm or more preferably lies in a visible light region, especially awavelength region from 400 to 800 nm.

Note that a reflection bandwidth is a reflection band having reflectanceof a half of the maximum reflectance in a reflectance spectrum of abroad band cholesteric liquid crystal film measured with aspectrophotometer (Instant Multiphotometry System Model No. MCPD-2000,manufactured by Otsuka Electronics Co., Ltd.).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a reflectance spectrum of a cholesteric liquid crystal filmmanufactured in Example 1.

FIG. 2 is a reflectance spectrum of a cholesteric liquid crystal filmmanufactured in Example 2.

FIG. 3 is a reflectance spectrum of a cholesteric liquid crystal filmmanufactured in Comparative Example 1.

FIG. 4 is a reflectance spectrum of a cholesteric liquid crystal filmmanufactured in Comparative Example 2.

FIG. 5 is a conceptual view of a broad band polarizing plate used inExample 3, wherein a numerical symbol (1) indicates an absorptionpolarizing plate, (2) a λ/4 plate, (3) a cholesteric liquid crystal film(circularly polarizing plate), (4) a pressure sensitive adhesive layer,(A1) a linearly polarizer and (A2) a linearly polarizer obtained bylaminating the absorption polarizing plate (1) on the linearly polarizer(A1).

FIG. 6 is a conceptual view of a wide viewing angle liquid crystaldisplay manufactured in Example 5, wherein a numerical symbol (1)indicates an absorption polarizing plate, (2) a λ/4 plate, (3) acholesteric liquid crystal film (reflection polarizer (a)), (5) aretardation plate (b): C plate, (6) a viewing angle magnification film(diffuse pressure sensitive adhesive), (LC) a liquid crystal cell, (BL)backlight, (D) diffusing reflective plate, (30) a polarizing element,(A1) a linearly polarizer and (A2) a linearly polarizer obtained bylaminating the absorption polarizing plate (1) on the linearly polarizer(A1).

BEST MODE FOR CARRYING OUT THE INVENTION

A cholesteric liquid crystal film of the present invention is obtainedby ultraviolet polymerizing a liquid crystal mixture containing apolymerizable mesogen compound (a), a polymerizable chiral agent (b) anda photoisomerizable material (c).

A polymerizable mesogen compound (a) preferably has at least onepolymerizable functional group and in addition, a mesogen groupcontaining a ring unit and others. As polymerizable functional groups,exemplified are an acryloyl group, a methacryloyl group, an epoxy group,a vinyl ether group and others, among which preferable are an acryloylgroup and a methacryloyl group. With a polymerizable mesogen compound(a) having two or more polymerizable functional groups employed, acrosslinked structure is introduced into a cholesteric liquid film tothereby enable durability thereof to be enhanced. Examples of the ringunit constituting a mesogen group include: a biphenyl-based ring unit, aphenylbenzoate-based ring unit, a phenylcyclohexane-based ring unit, anazoxybenzene-based ring unit, an azomethine-based ring unit, anazobenzene-based ring unit, a phenylpyrimidine-based ring unit, adiphenylacetylene-based ring unit, a diphenylbenzoate-based ring unit, abicyclohexane-based ring unit, a cyclohexylbenzene-based ring unit, aterphenyl-based ring unit and others. An end of each of the ring unitsmay has any of substituents such as a cyano group, an alkyl group, analkoxy group, a halogen atom. A mesogen group described above may couplewith a spacer portion imparting a bendability interposed between thegroups itself. As spacer portions, exemplified are a polyethylene chain,a polyoxymethylene chain and others. The number of repeated structuralunits constituting a spacer portion is properly determined according toa chemical structure of a mesogen moiety, wherein the number ofrepetition units in a polymethylene chain ranges from 0 to 20 andpreferably from 2 to 12 and the number of repetition units in apolyoxymethylene chain ranges from 0 to 10 and preferably 1 to 3.

As a polymerizable mesogen compound (a) having one polymerizablefunctional group, exemplified is a compound expressed by the followingformula:

, wherein R₁ indicates a hydrogen atom or a methyl group and n aninteger from 1 to 5.

As concrete examples of the polymerizable mesogen compound (a) havingone polymerizable functional group, exemplified are the followingcompounds:

As concrete examples of polymerizable mesogen compounds (a) having twopolymerizable functional groups, exemplified are LC242 manufactured byBASF Corp.

As a polymerizable chiral agent (b), exemplified is LC756 manufacturedby BASF Corp.

A mixing amount of a polymerizable chiral agent (b) is preferably on theorder in the range of from 1 to 20 parts by weight and more preferablyin the range of from 3 to 7 parts by weight relative to 100 parts byweight of a total amount of a polymerizable mesogen compound (a) and thepolymerizable chiral agent (b). A helical twist power (HTP) iscontrolled by a ratio of a polymerizable mesogen compound (a) and apolymerizable chiral agent (b). By adjusting the proportion within therange, a reflection band can be selected so that a reflectance spectrumof an obtained cholesteric liquid crystal film can cover all the rangeof visible light.

As a photoisomerizable material (c), any of compounds causing aphotoisomerization reaction can be employed without imposing anyspecific limitation thereon. Examples of photoisomerizable materials (c)include compounds such as stilbene, stilbenes, azobenzene, azobenzenes,spiropyrans, spirooxazines, diaryl ethers, filgides, cyclophanes,calcons and others. As a photoisomerizable material, among them, it ispreferable to use at least one kind selected from the group consistingof stilbene, azobenzene and derivatives thereof. An added amount of aphotoisomerizable material (c) is not particularly limited, butpreferably on the order in the range of 0.1 to 20 parts by weight andmore preferably in the range of from 2 to 10 parts by weight relative to100 parts by weight of a total amount of a polymerizable mesogencompound (a) and a polymerizable chiral agent (b).

Any kind of photopolymerization initiators (d) can be employed withoutimposing any specific limitation thereon. Exemplified are IRGACURE 184,IRGACURE 907, IRGACURE 369, IRGACURE 651 and others manufactured byChiba Specialty Chemicals Corp. A mixing amount of a photopolymerizationinitiator is preferably on the order in the range of from 0.01 to 10parts by weight and more preferably in the range of from 0.05 to 5 partsby weight relative to 100 parts by weight of a total amount of apolymerizable mesogen compound (a) and a polymerizable chiral agent (b).Note that a photopolymerization initiator (d) is not necessarily addeddepending on ultraviolet illumination conditions or an added amount of aphotoisomerizable material (c). For example, in a case where apolymerizable mesogen compound (a) and a polymerizable chiral agent (b)each having two polymerizable functional groups are combined and asufficient fast reaction speed is obtained as expected in thecombination, no photopolymerization initiator (d) is required to beadded.

In the present invention, a liquid crystal mixture containing apolymerizable mesogen compound (a), a polymerizable chiral agent (b) anda photoisomerizable material (c), and a photopolymerization initiator(d) when required, can be used as a solution obtained by dissolving themixture into a solvent. Without a specific limitation imposed,preferable as solvents used are methyl ethyl ketone, cyclohexanone,cyclopentanone and others. A concentration of a solution is usually onthe order in the range of from 3 to 50 weight %.

Manufacture of a cholesteric liquid crystal film of the presentinvention is implemented by coating the liquid crystal mixture on asubstrate, followed by ultraviolet polymerization.

As substrates, there can be adopted conventionally known members asones. Exemplified are: a rubbing film obtained by subjecting a thin filmmade of polyimide, polyvinyl alcohol or the like formed on a substrateto a rubbing treatment with rayon cloth; an obliquely deposition film;optically oriented film obtained by illuminating a polymer havingphotocrosslinking group such as cynnamate, azobenzene or the like or apolyimide with polarized ultraviolet; and a stretched film and others.Orientation can be implemented by application of a magnetic field, anelectric field and a shearing stress.

Examples of the substrate that are used include: films made of plasticssuch as polyethylene phthalate, triacetyl cellulose, norbornen resin,polyvinyl alcohol, polyimide, polyallylate, polycarbonate, polysulfone,polyethersulfone and others; a glass plate, a quartz sheet and others.

A liquid crystal mixture described above is coated on one substrate andthereafter, the other substrate can be laminated on the coat. In casewhere the liquid crystal mixture is a solution, the solution is coatedon one substrate and the coat is dried, followed by laminating the othersubstrate on the coat. A drying temperature for evaporating a solventhas only to be a temperature of the boiling temperature of the solventor higher. The temperature is only required to be set usually in therange of 80 to 160° C. according to a kind of a solvent.

A thickness of a coat of a liquid crystal mixture described above (in acase of a solution, a thickness of a coat in a dry state after a solventis evaporated) is preferably on the order in the range of from 1 to 20μm and more preferably on the order in the range of 2 to 10 μm. If acoat thickness is less than 1 μm, there rises an unprofitable tendencyto decrease a polarization degree though a reflection bandwidth issecured. On the other hand, if the thickness is more than 20 μm, more ofimprovement is unprofitably not realized on a reflection bandwidth orpolarization degree only to increase a cost.

An ultraviolet illuminance is preferably in the range of from 0.1 to 30mW/cm² and more preferably in the range of from 1 to 20 mW/cm². Anillumination time is preferably a shorter time of 5 min or less, morepreferably 3 min or less and furthers more preferably 1 min or less.Note that if heating is applied on the opposite side from an ultravioletillumination side, a broad band cholesteric liquid crystal film can berealized in a shorter time.

A heating temperature, on or after ultraviolet illumination, has only tobe a liquid crystal temperature or higher and usually preferably 140° C.or lower in a general practice. The temperature is, to be concrete,preferably on the order in the range of from 60 to 140° C. and morepreferably in the range of 80° C. to 120° C. With heating applied, aneffect is exerted that a diffusion speed of a monomer component isaccelerated. If the temperature is lower than 60° C., a diffusion speedof a polymerizable mesogen compound (a) is very slow, requiring a verylong time in order to achieve a broad band.

A thus obtained cholesteric liquid crystal film may be used either notbeing separated from a substrate or being separated therefrom.

A broad band cholesteric liquid crystal film of the present invention isused as a circularly polarizing plate. A circularly polarizing platewith a λ/4 plate laminated thereon can be used as a linearly polarizer.A cholesteric liquid crystal film, which is a circularly polarizingplate, is preferably laminated on a λ/4 plate so that a pitch length inthe film is narrowed toward the λ/4 plate continuously.

As λ/4 plates, exemplified are: a birefringent film obtained bystretching a film made of a proper plastic such as polycarbnate,norbornen resin, polyvinyl alcohol, polystyrene, polymethylmethacrylate,polypropylene, other polyolefins, polyallylate, polyimide; a alignedfilm made of a liquid crystal material such as a liquid crystal polymer;an aligned layer of a liquid crystal material supported by a film; andothers. A thickness of a λ/4 plate is usually preferably in the range offrom 0.5 to 200 μm and especially preferably in the range of from 1 to100 μm.

A retardation plate functioning as a λ/4 plate in a broad wavelengthrange such as a visible light region can be obtained, for example, by ascheme to superimpose a retardation layer functioning as a λ/4 plate fora monochromatic light of wavelength of 550 nm and a retardation layerexhibiting another retardation characteristic, for example a retardationlayer functioning as a λ/2 plate on each other or the like scheme.Therefore, a retardation plate arranged between a polarizing plate and abrightness enhancement improving film may be made of either oneretardation layer, or two or more retardation layers.

An absorption polarizer is adhered to the linearly polarizer, so that atransmission axis direction of the linearly polarizer are arranged inparallel with each other.

The polarizer is not limited especially but various kinds of polarizermay be used. As a polarizer, for example, a film that is uniaxiallystretched after having dichromatic substances, such as iodine anddichromatic dye, absorbed to hydrophilic high molecular weight polymerfilms, such as polyvinyl alcohol type film, partially formalizedpolyvinyl alcohol type film, and ethylene-vinyl acetate copolymer typepartially saponified film; poly-ene type orientation films, such asdehydrated polyvinyl alcohol and dehydrochlorinated polyvinyl chloride,etc. may be mentioned. In these, a polyvinyl alcohol type filmcontaining dichromatic materials such as iodine is suitably used.Although thickness of polarizer is not especially limited, the thicknessof about 5 to 80 μm is commonly adopted.

A polarizer that is uniaxially stretched after a polyvinyl alcohol typefilm dyed with iodine is obtained by stretching a polyvinyl alcohol filmby 3 to 7 times the original length, after dipped and dyed in aqueoussolution of iodine. If needed the film may also be dipped in aqueoussolutions, such as boric acid and potassium iodide. Furthermore, beforedyeing, the polyvinyl alcohol type film may be dipped in water andrinsed if needed. By rinsing polyvinyl alcohol type film with water,effect of preventing un-uniformity, such as unevenness of dyeing, isexpected by making polyvinyl alcohol type film swelled in addition thatalso soils and blocking inhibitors on the polyvinyl alcohol type filmsurface may be washed off. Stretching may be applied after dyed withiodine or may be applied concurrently, or conversely dyeing with iodinemay be applied after stretching. Stretching is applicable in aqueoussolutions, such as boric acid and potassium iodide, and in water bath.

A polarizing plate on which a transparent protective film prepared onone side or both sides of the polarizer is used. Materials of thetransparent protective, excellent in transparency, mechanical strength,heat stability, water shielding property, isotropy etc., is may bepreferably used, As transparent protective films, for example,transparent polymer films made of polyester type polymers, such aspolyethylene terephthalate and polyethylenenaphthalate; cellulose typepolymers, such as diacetyl cellulose and triacetyl cellulose;polycarbonate type polymer; acrylics type polymer, such as polymethylmethacrylate may be mentioned. Besides, as examples of thetransparent polymer films made of styrene type polymers, such aspolystyrene and acrylonitrile-styrene copolymer; polyolefin typepolymers, such as polyethylene, polypropylene, polyolefin that hascyclo-type or norbornen structure, ethylene-propylene copolymer; vinylchloride type polymer; amide type polymers, such as nylon and aromaticpolyamide may be mentioned. Besides, as examples of the transparentpolymer films made of imide type polymers; sulfone type polymers;polyether sulfone type polymers; polyether-ether ketone type polymers;poly phenylene sulfide type polymers; vinyl alcohol type polymer;vinylidene chloride type polymers; vinyl butyral type polymers; allylatetype polymers; polyoxymethylene type polymers; epoxy type polymers; orblend polymers of the above-mentioned polymers may be mentioned.Especially, preferable when being used is a film made of a transparentpolymer with less of optical birefringence. Preferable from theviewpoint of a protective film for a polarizing plate are triacetylcellulose, polycarbonate, acrylics type polymer, a cyclo-olefine typeresin, polyolefin having a norbornen structure and others.

Moreover, as is described in Japanese Patent Laid-Open Publication No.2001-343529 (WO 01/37007), polymer films, for example, resincompositions including (A) thermoplastic resins having substitutedand/or non-substituted imido group is in side chain, and (B)thermoplastic resins having substituted and/or non-substituted phenyland nitrile group in sidechain may be mentioned. As an illustrativeexample, a film may be mentioned that is made of a resin compositionincluding alternating copolymer comprising iso-butylene and N-methylmaleimide, and acrylonitrile-styrene copolymer. A film comprisingmixture extruded article of resin compositions etc. may be used.

As a transparent protective film preferably used, in viewpoint ofpolarization property and durability, triacetyl cellulose film whosesurface is saponificated with alkali is suitable. In general, athickness of a transparent protective film is about 10 through 500 μm,preferably 20 through 300 μm, and especially preferably 30 through 200μm.

Moreover, it is preferable that the transparent protective film may haveas little coloring as possible. Accordingly, a protective film having aretardation value in a film thickness direction represented byRth=[(nx+ny)/2−nz]×d of −90 nm through +75 nm (where, nx and nyrepresent principal indices of refraction in a film plane, nz representsrefractive index in a film thickness direction, and d represents a filmthickness) may be preferably used. Thus, coloring (optical coloring) ofpolarizing plate resulting from a protective film may mostly becancelled using a protective film having a retardation value (Rth) of−90 nm through +75 nm in a thickness direction. The retardation value(Rth) in a thickness direction is preferably −80 nm through +60 nm, andespecially preferably −70 nm through +45 nm.

The transparent protective films on the front and back sides may also betransparent protective films made of either the same polymer material orrespective different polymer materials.

A hard coat layer may be prepared, or antireflection processing,processing aiming at sticking prevention, diffusion or anti glare may beperformed onto the face on which the polarizing film of the abovedescribed transparent protective film has not been adhered.

A hard coat processing is applied for the purpose of protecting thesurface of the polarizing plate from damage, and this hard coat film maybe formed by a method in which, for example, a curable coated film withexcellent hardness, slide property etc. is added on the surface of theprotective film using suitable ultraviolet curable type resins, such asacrylic type and silicone type resins. Antireflection processing isapplied for the purpose of antireflection of outdoor daylight on thesurface of a polarizing plate and it may be prepared by forming anantireflection film according to the conventional method etc. Besides, asticking prevention processing is applied for the purpose of adherenceprevention with adjoining layer.

In addition, an anti glare processing is applied in order to prevent adisadvantage that outdoor daylight reflects on the surface of apolarizing plate to disturb visual recognition of transmitting lightthrough the polarizing plate, and the processing may be applied, forexample, by giving a fine concavo-convex structure to a surface of theprotective film using, for example, a suitable method, such as roughsurfacing treatment method by sandblasting or embossing and a method ofcombining transparent fine particle. As a fine particle combined inorder to form a fine concavo-convex structure on the above-mentionedsurface, transparent fine particles whose average particle size is 0.5to 50 μm, for example, such as inorganic type fine particles that mayhave conductivity comprising silica, alumina, titania, zirconia, tinoxides, indium oxides, cadmium oxides, antimony oxides, etc., andorganic type fine particles comprising cross-linked of non-cross-linkedpolymers may be used. When forming fine concavo-convex structure on thesurface, the amount of fine particle used is usually about 2 to 50weight part to the transparent resin 100 weight part that forms the fineconcavo-convex structure on the surface, and preferably 5 to 25 weightpart. An anti glare layer may serve as a diffusion layer (viewing anglemagnifying function etc.) for diffusing transmitting light through thepolarizing plate and magnifying a viewing angle etc.

In addition, the above-mentioned antireflection layer, stickingprevention layer, diffusion layer, anti glare layer, etc. may be builtin the protective film itself, and also they may be prepared as anoptical layer different from the protective layer.

The linearly polarizer described above can be provided with a pressuresensitive adhesive layer for adhering itself to another member such as aliquid crystal cell or the like. As pressure sensitive adhesive thatforms adhesive layer is not especially limited, and, for example,acrylic type polymers; silicone type polymers; polyesters,polyurethanes, polyamides, polyethers; fluorine type and rubber typepolymers may be suitably selected as a base polymer. Especially, apressure sensitive adhesive such as acrylics type pressure sensitiveadhesives may be preferably used, which is excellent in opticaltransparency, showing adhesion characteristics with moderatewettability, cohesiveness and adhesive property and has outstandingweather resistance, heat resistance, etc.

In addition to the above description, a pressure sensitive adhesivelayer low in moisture absorption rate and excellent in heat resistanceis preferable from the viewpoints of prevention of a foaming phenomenonand peeling-off phenomenon due to moisture absorption, prevention ofdegradation in optical characteristic and warp of a liquid crystal celldue to the difference of thermal expansion or the like and further, inconsideration of formability of a high quality liquid crystal displayexcellent in durability and the like.

The pressure sensitive adhesive layer may contain additives, forexample, such as natural or synthetic resins, tackifier, glass fibers,glass beads, metal powder, fillers comprising other inorganic powderetc., pigments, colorants and antioxidants. Moreover, it may be apressure sensitive adhesive layer that contains fine particle and showsoptical diffusion nature.

Proper method may be carried out to attach a pressure sensitive adhesivelayer. As an example, about 10 to 40 weight % of the pressure sensitiveadhesive solution in which a base polymer or its composition isdissolved or dispersed, for example, toluene or ethyl acetate or a mixedsolvent of these two solvents is prepared. A method in which thissolution is directly applied on a polarizer using suitable developingmethods, such as flow method and coating method, or a method in which anadhesive layer is once formed on a separator, as mentioned above, and isthen transferred on a an optical film may be mentioned. A pressuresensitive adhesive layer may be prepared with two or more layers whichare made of different compositions or kinds with each layer. Thicknessof an adhesive layer may be suitably determined depending on a purposeof usage or adhesive strength, etc., and generally is 1 to 500 μm,preferably 5 to 200 μm, and more preferably 10 to 100 μm.

A temporary separator is attached to an exposed side of a pressuresensitive adhesive layer to prevent contamination etc., until it ispractically used. Thereby, it can be prevented that foreign mattercontacts adhesive layer in usual handling. As a separator, withouttaking the above-mentioned thickness conditions into consideration, forexample, suitable conventional sheet materials that is coated, ifnecessary, with release agents, such as silicone type, long chain alkyltype, fluorine type release agents, and molybdenum sulfide may be used.As a suitable sheet material, plastics films, rubber sheets, papers,cloths, no woven fabrics, nets, foamed sheets and metallic foils orlaminated sheets thereof may be used.

In addition, ultraviolet absorbing property may be given to theabove-mentioned each layer, such as a pressure sensitive adhesive layer,using a method of adding UV absorbents, such as salicylic acid estertype compounds, benzophenol type compounds, benzotriazol type compounds,cyano acrylate type compounds, and nickel complex salt type compounds.

A linearly polarizer of the present invention can be preferably used inmanufacture of various kinds of apparatuses such as a liquid crystaldisplay and others. Assembling of a liquid crystal display may becarried out according to conventional methods. That is, a liquid crystaldisplay is generally manufactured by suitably assembling several partssuch as a liquid crystal cell, optical elements and, if necessity,lighting system, and by incorporating driving circuit. In the presentinvention, except that a linearly polarizer by the present invention isused, there is especially no limitation to use any conventional methods.Also any liquid crystal cell of arbitrary type, such as TN type, and STNtype, π type may be used.

Suitable liquid crystal displays, such as liquid crystal display withwhich the above-mentioned linearly polarizer has been located at oneside or both sides of the liquid crystal cell, and with which abacklight or a reflector is used for a lighting system may bemanufactured. In this case, the linearly polarizer by the presentinvention may be installed in one side or both sides of the liquidcrystal cell. When installing the linearly polarizers in both sides,they may be of the same type or of different type. Furthermore, inassembling a liquid crystal display, suitable parts, such as diffusionplate, anti-glare layer, antireflection film, protective plate, prismarray, lens array sheet, optical diffusion plate, and backlight, may beinstalled in suitable position in one layer or two or more layers.

A circularly polarizing plate (a reflection polarizer) using acholesteric liquid crystal film described above is used in a polarizingelement system in which a retardation layer (b) having a front faceretardation (in the normal direction) of almost zero and a retardationof λ/8 or more relative to an incident light incoming at an angle of 30°C. or more inclined from the normal direction is arranged between atleast two layer reflection polarizer (a) with respective selectivereflection wavelength bands of polarized light superimposed on eachother. Note that a cholesteric liquid crystal film is of a constructionin which any of the sides of the maximum pitch of a helically twistedmolecular structure and the minimum pitch thereof may be located closerto the retardation layer (b), while if a reflection polarizer (a) isexpressed by (maximum pitch/minimum pitch), arrangement thereof ispreferably in a structure of maximum pitch/minimum pitch/retardationlayer (b)/maximum pitch/minimum pitch from the view point of a viewingangle (in other words, a better viewing angle and less of coloringabnormality). In a case where a λ/4 plate is combined as shown in FIG.6, the minimum pitch side of a reflection polarizer (a) is preferablyarranged closer to the λ/4 plate.

The polarizing element system, that is a cholesteric liquid crystallaminate having a broad band selective reflection function, has acircularly polarizing light reflection/transmission function in thefront face direction and can be used in a liquid crystal display as abroad band circularly polarizing plate. In this case, the cholestericliquid crystal laminate is arranged on the light source side of a liquidcrystal cell in a circularly polarizing mode, for example a transmissiontype VA mode liquid crystal cell having a multidomain and thereby can beused as a circularly polarizing plate.

The retardation layer (b) has a retardation of almost zero in the frontface direction and a retardation of λ/8 or more relative to an incidentlight incoming at an angle of 30° inclined from the normal direction.The front face retardation works for holding polarized light vertically,which is desirably λ/10 or less.

In order to effectively polarization-convert an incident light incomingin an oblique direction, a retardation of the incident light is properlydetermined by an angle at which the light is totally reflected. Forexample, in order to totally reflect an incident light at an angle ofthe order of 60° inclined from the normal, retardation as measured at60° has only to be determined so as to be a value of the order of λ/2.Since transmitted light through the reflection polarizer (a) ismodulated with respect to a polarization state thereof even due tobirefringence like a C plate of the reflection polarizer itself, aretardation as measured at the angle of a C plate inserted may beusually a value smaller than λ/2. Since a retardation of the C platemonotonously increases with an inclination of an incident light, aretardation has only to be λ/8 or more relative to an incident lightincoming at an angle of 30° as a target at which effective totalreflection is caused at an angular inclination of 30° or more.

A material of the retardation layer (b) may be any of materials havingthe above described optical property without any specific limitationthereon. Examples thereof include: a layer having a fixed planaralignment state of a cholesteric liquid crystal having a selectivereflection wave length other than the visible light region (from 380 nmto 780 nm); a layer having a fixed homeotropic alignment state of arod-like liquid crystal; a layer using a columnar alignment or a nematicalignment of a discotic liquid crystal; a layer aligned a negativeuniaxial crystal in-plane; a biaxially aligned polymer film; and others.

In the present invention, a C plate in which fixed is a planar alignmentstate of a cholesteric liquid crystal having a selective reflectionwavelength other than the visible light region (from 380 nm to 780 nm)desirably has no coloring abnormality in the visible light region as aselective reflection wavelength of a cholesteric liquid crystal.Therefore, a necessity arises for no selective reflection light in thevisible light region. Selective reflection is specifically determined bya cholesteric chiral pitch and a refractive index of a liquid crystal.While a value of a central wavelength of selective reflection may be inthe near infrared region, the value is desirably in an ultravioletregion of 350 nm or less in wavelength because of an influence ofrotatory polarization and generation of slightly complex phenomenon.Formation of a cholesteric liquid crystal layer is conducted in asimilar way to that in formation of a cholesteric layer in a reflectionpolarizer described above.

A C plate having a fixed homeotropic alignment state in the presentinvention is made of a polymer liquid crystal having been obtained bypolymerizing a liquid crystalline thermoplastic resin exhibiting anematic liquid crystallinity at high temperature or a liquid crystalmonomer and an alignment agent, when required, with an ionizingradiation such as an electron beam, ultraviolet or the like; or heating;or a mixture of polymer liquid crystals. While a liquid crystallinitymay be either lyotropic or thermotropic, a thermotropic liquid crystalis desirable from the view point of ease of control and formability ofmonodomain. A homeotropic alignment is obtained for example in aprocedure in which a birefringent material described above is coated ona film made of a vertical aligned film (such as a film of a long chainalkylsilane) and a liquid crystal state is revealed in the film andfixed.

As a C plate using a discotic liquid crystal, there is available a plateobtained by revealing and fixing a nematic phase or a columnar phase ofa triphenylene compounds each having an in-plane spread molecule as aliquid crystal material or a discotic liquid crystal material having anegative uniaxiality such as phthalocyanines. Inorganic layeredcompounds each with a negative uniaxiality are detailed in thepublication of JP-A No. 6-82777 and others.

A C plate using a biaxial alignment of a polymer film can be obtained byone of the following methods, in which a polymer film having a positiverefractive index anisotropy is biaxially stretched in a good balance; inwhich a thermoplastic resin is pressed; in which a C plate is cut offfrom a parallel aligned crystal; and in others.

While lamination of layers may be only as superimposed, the laminationis desirable to use an adhesive or a pressure sensitive adhesive fromthe viewpoint of workability and light utilization efficiency. In thecase, it is desirable that an adhesive or a pressure sensitive adhesiveis transparent and has no absorption in the visible light region, and arefractive index is as equal to each of refractive indexes of the otherlayers as possible from the viewpoint of suppression of surfacereflection. From the viewpoint, for example, an acrylic pressuresensitive adhesive or the like is preferably used. Lamination of layerscan be implemented according to the following methods: in which layersare formed in monodomains separately as respective aligned films andsequentially layered onto a translucent substrate by transfer or thelike scheme; and in another of which aligned films are properly formedfor alignment without providing adhesive layers and the layers aresequentially formed directly on a previous layer.

Other procedures can be adopted: in which particles are further addedonto layers and adhesive or pressure sensitive adhesive layers foradjustment of a diffuse level, when required, to thereby impart anisotropic scatterability; and in another of which properly added are anultraviolet absorbent, an antioxidant and a surfactant for the purposeto impart a leveling property on film formation.

While a polarizing element (a cholesteric liquid crystal laminate) ofthe present invention has a circularly polarized lightreflection/transmission function, the element is combined with a λ/4plate to thereby convert a transmitted light to a linearly polarizedlight and enable it to be used as a linearly polarizer.

Examples of λ/4 plates that are properly used are not particularlylimited, but include: a general purpose transparent resin filmgenerating a retardation by stretching such as films made ofpolycarbonate, polyethylene terephthalate, polystyrene, polysulfone,polyvinyl alcohol, polymethylmethacrylate or the like; a norbornen resinfilm such as an ARTON film manufactured by JSR; and others. Furthermore,biaxial stretching is applied and a retardation plate compensating achange in retardation caused by an incidence angle is used to therebyenable a viewing angle characteristic to be improved, which ispreferable. There may also be used a λ/4 plate obtained by fixing a λ/4layer prepared by aligning a liquid crystal which reveals no retardationby resin stretching. In this case, a thickness of a λ/4 plate can begreatly reduced. A thickness of a λ/4 plate is usually preferably in therange of from 0.5 to 200 μm and especially preferably in the range offrom 1 to 100 μm.

While a λ/4 plate well works only for a specific wavelength in a case ofa single layer made of a single material, a problem for otherwavelengths arises that a function as a λ/4 plate is degraded withrespect to a wavelength dispersing characteristic for other wavelengths.Therefore, by laminating while defining an axial angle relative to a λ/2plate, a λ/4 plate can be used as a broad band λ/4 plate functioning ina range in which no practical inconvenience arises in the entire visiblelight region. A λ/4 plate and a λ/2 plate in this case may be made withthe same material, or a λ/4 plate is made with a different materialobtained in a similar way to that in the case of the λ/4 plate describedabove and may be combined with the λ/2 plate.

For example, a λ/4 plate (140 nm) is laminated on a broad bandcircularly polarizing plate and a λ/2 plate (270 nm) is disposed at117.5° relative to an axial angle of the λ/4 plate. A transmissionpolarization axis is 10° relative to the axis of the λ/4 plate. Sincethe adhering angle changes according to a retardation value of eachretardation plate, the adhering angle is not specifically limited.

An absorption polarizer is adhered to the linearly polarizer so that atransmission axis direction of the absorption polarizer and atransmission axis of the linearly polarizer are arranged in parallelwith each other.

Arrangement of Diffusing Reflective Plate

A diffusing reflective plate is desirably arranged at the down side (theother side from an arrangement surface of a liquid cell) of a lightguide plate as a light source. A main component of light reflected by acollimate film is an oblique incident light component and the maincomponent of light is specular reflected by the collimate film andreflected back in the backlight direction. On this occasion, in a casewhere a reflective plate on the back surface side is high in specularreflection, a reflective angle is retained and cannot be emitted in thefront face direction only to end up with light loss. Therefore, areflective angle of reflected-back light is not retained to therebyincrease a scattering reflection component in the front face direction;therefore the arrangement of a diffusing reflective plate is desirable.

Arrangement of Diffuse Plate

It is also desirable to place a proper diffuse plate between a collimatefilm in the present invention and the backlight source. This is becauselight impinged obliquely and reflected is scattered in the vicinity of abacklight guide plate and part of the reflected light is scattered inthe vertical incidence direction to thereby enhance a second utilizationof light.

A diffuse plate used can be obtained by means of a method in which anunevenness surface utilized, or in which particles with differentrefractive indexes are embedded in a resin. The diffuse plate either maybe inserted between a collimate film and a backlight or may be adheredto a collimate film.

In a case where a liquid crystal cell to which a collimate film isadhered is placed adjacent to a backlight, there is a chance to cause aNewton ring in a clearance between a film surface and the backlight,while by placing a diffuse plate having an unevenness surface on thelight guide plate side surface of the collimate film in the presentinvention, it can be suppressed to generation of Newton ring. Moreover,a layer serving as an unevenness surface and a light diffusing structuremay be formed as a surface itself of a light parallel film in thepresent invention.

Arrangement of View Angle Magnifying Film

Magnification of a viewing angle in a liquid crystal display of thepresent invention can be achieved by obtaining a uniform and gooddisplay characteristic all over the viewing angle through diffusinglight of good display characteristic in the vicinity of the front faceobtained from a liquid crystal display combined with a collimatedbacklight.

A viewing angle magnification film used here is a diffuse plate havingsubstantially no backscattering. A diffuse plate can be provided as adiffusing pressure sensitive adhesive material. An arranging placethereof can be used up or down of a polarizing plate on the viewer sideof the liquid crystal display. In order to prevent reduction in contrastdue to an influence such as blotting of pixels or a slightly remainingbackscattering, the diffuse plate is desirably placed in a layer at aposition the closest possible to a cell such as between a polarizingplate and a liquid crystal cell. In this case, it is desirable to use afilm that does not substantially cancel polarization. A fine particledistribution type diffuse plate is preferably used, which is disclosedin, for example, the publications of JP-A No. 2000-347006 and JP-A No.2000-347007.

In a case where a viewing angle magnification film is disposed outsideof a polarizing plate, a viewing angle compensating retardation platemay not be used especially if a TN liquid cell is used since collimatedlight is transmitted through a liquid crystal layer and through thepolarizing plate. If an STN liquid crystal cell is used in the case, ithas only to use a retardation plate that is well compensated withrespect to a front face characteristic. Since, in this case, a viewingangle magnification film has a surface exposed to the air; a type havinga refractive effect due to a surface profile can also be employed.

On the other hand, in a case where a viewing angle magnification film isinserted between a polarizing plate and a liquid crystal layer, light isdiffuse light at the stage where light is transmitted through thepolarizing plate. If a TN liquid crystal is used, a necessity arises forcompensating a viewing angle characteristic of the polarizer itself. Inthis case, it is necessary to insert a retardation plate to compensate aviewing angle characteristic of a polarizer between the polarizer andthe viewing angle magnification film. If an STN liquid crystal is used,it is necessary to insert a retardation plate to compensate a viewingangle characteristic of the polarizer in addition to a front faceretardation compensation for the STN liquid crystal.

In a case of a viewing angle magnification film having a regularstructure in the interior thereof such as a microlens array film or ahologram film, both conventionally having been available, interferencehas occurred with a fine structure such as a microlens array, a prismarray, a louver, a micromirror array or the like that is included in ablack matrix of a liquid crystal display or a collimate system of aconventional backlight to thereby cause a moiré pattern with ease. Sincein a collimate film in the present invention, a regular structure is notvisually recognized in a plane thereof and emitting light has noregularity modulation, no necessity arises for consideration of matchingwith a viewing angle magnification film or an arrangement sequence.Therefore, a viewing angle magnification film has a lot of options sinceno specific limitation is imposed thereon as far as neither interferencenor a moiré pattern occurs with a pixel black matrix of a liquid crystaldisplay.

In the present invention, as viewing angle magnification films,preferably used are a light scattering plate, having no substantialbackscattering and not canceling polarization, which is described in anyof the publications of JP-A Nos. 2000-347006 and 2000-347007 and whichhas a haze in the range of 80% to 90%. Any of films each of which has aregular structure in the interior thereof such as a hologram sheet, amicroprism array, a microlens array or the like can be used as far asneither interference nor a moiré pattern occurs with a pixel blackmatrix of a liquid crystal display.

Note that for use in a liquid crystal display, various kinds of opticallayers are properly prepared according to ordinary methods.

EXAMPLES

Description will be given of the present invention showing examples andcomparative examples below, while the present invention is not limitedto the examples.

Example 1

Prepared was a methyl ethyl ketone solution (with a solid matter contentof 30 wt %) of a mixture composed of 96 parts by weight of LC242manufactured by BASF Corp. as a polymerizable mesogen compound (a), 4parts by weight of LC756 manufactured by BASF Corp. as a polymerizablechiral agent (b) and 5 parts by weight of stilbene as aphotoisomerizable material (c). The solution was cast on one stretchedpolyethylene terephthalate substrate and a solvent was removed off at100° C. for 2 min for drying, followed by laminating the otherpolyethylene terephthalate substrate thereon. Then, the laminate wasapplied with ultraviolet illumination at 5 mW/cm² for 3 min and furtherwith heating at 100° C. for 10 sec, thereby obtaining a cholestericliquid crystal film as a target.

The one polyethylene terephthalate substrate was removed. A reflectancespectrum of a cholesteric liquid crystal film (a circularly polarizingplate) is shown in FIG. 1. The circularly polarizing plate had a goodcircularly polarized light separating characteristic (a reflection band)in the wavelength range of from 400 to 800 nm. A total thickness of thecholesteric liquid crystal layer (film) was 10 μm. A pitch length in theobtained cholesteric liquid crystal layer was 0.2 μm in the vicinity ofan ultraviolet illuminated surface (in the lower layer at 1 μm below theultraviolet illuminated surface), while being 0.5 μm in the vicinity ofthe opposite surface (in the lower layer at 1 μm below the oppositesurface). A pitch length was measured with a sectional TEM photograph. Abroad band cholesteric liquid crystal film covering visible light wasable to be manufactured as a single layer in this way.

Example 2

Prepared was a methyl ethyl ketone solution (with a solid matter contentof 20 wt %) of a mixture composed of 96 parts by weight of the abovedescribed compound (1) as a polymerizable mesogen compound (a), 4 partsby weight of LC756 manufactured by BASF Corp. as a polymerizable chiralagent (b), 5 parts by weight of azobenzene as a photoisomerizablematerial (c) and 5 parts by weight of IRGACURE 369 (manufactured byChiba Specialty Chemicals Corp.) as a photopolymerization initiator (d).The solution was cast on one stretched polyethylene terephthalatesubstrate and a solvent was removed off at 100° C. for 2 min for drying.Then, the laminate was applied with ultraviolet illumination at 20mW/cm² for 10 sec and further with heating, thereby obtaining acholesteric liquid crystal film as a target.

A reflectance spectrum of a cholesteric liquid crystal film (acircularly polarizing plate) is shown in FIG. 2. The obtained circularlypolarizing plate had a good circularly polarized light separatingcharacteristic in the wavelength range of from 450 to 900 nm). A totalthickness of the cholesteric liquid crystal layer (film) was 6 μm. Apitch length in the obtained cholesteric liquid crystal layer was 0.25μm in the vicinity of an ultraviolet illuminated surface (in the lowerlayer at 1 μm below the ultraviolet illuminated surface), while being0.6 μm in the vicinity of the opposite surface (in the lower layer at 1μm below the opposite surface). A broad band cholesteric liquid crystalfilm covering visible light was able to be manufactured as a singlelayer in this way.

Example 3

The broad band cholesteric liquid crystal film (a circularly polarizingplate) obtained in Example 1 was adhered to a λ/4 plate obtained bybiaxially stretching a polycarbonate resin film (a thickness of 80 μm)in the direction along which a pitch length is narrower continuouslytoward the λ/4 plate with an acrylic pressure sensitive adhesive with athickness of 25 μm. Furthermore, an absorption type polarizing plateTEG1465DU manufactured by NITTO DENKO CO., LTD. was adhered thereto sothat the transmission axis directions coincide with each other, toobtain a broad band polarizing plate.

The broad band polarizing plate was used as a lower plate for a TFT-LCDand placed on a side light type backlight, and a brightness enhancementpercentage was measured with the result of a brightness enhancement 1.3or more times as that in a case where a product of the present inventionis not used. A brightness was measured with a viewing angle measuringinstrument EZ-CONTRAST manufactured by ELDIM Corp. Note that theobtained optical characteristic (a reflectance spectrum) was equal inperformance to a case where a film obtained in the above known patentliteratures was used.

Example 4

Prepared was a cyclopentanone solution (with a solid matter content of30 wt %) of a mixture composed of 88.6 parts by weight of aphotopolymerizable nematic liquid crystal monomer (manufactured by BASFCorp. with a trade name of LC242), 11.4 parts by weight of a chiralagent (manufactured by BASF Corp. with a trade name of LC756) and 5parts by weight of a photopolymerization initiator (manufactured byChiba Specialty Chemicals Corp. with a trade name of IRGACURE 907). Thesolution was mixed for adjustment so that a selective reflectionwavelength is 350 nm. The solution was coated on a polyethyleneterephthalate substrate to a thickness after drying of 4 μm using a wirebar and a solvent was removed off for drying. Thereafter, the film wastemporarily heated to an isotropic transition temperature of the liquidcrystal monomer and thereafter, gradually cooled to form a layer in auniformly oriented state. The obtained layer was illuminated withultraviolet to fix an aligned state and obtain a C plate (negative). Aretardation of the C plate was measured to be 2 nm in the front facedirection and 100 nm in a direction oblique by 30° for light having awavelength of 550 nm.

On the other hand, two broad band cholesteric liquid films (circularlypolarizing plates that is reflection polarizers) obtained in Example 1were prepared. The C plate layer was transferred on the reflectionpolarizer layer using a translucent adhesive. The same reflectionpolarizer layer was transferred and laminated on the C plate using atranslucent adhesive layer to obtain a polarizing element. A λ/4 platemade of a biaxially stretched polyethylene terephthalate was adhered tothe polarizing element so that the transmission axis coincides with thatof the polarizing plate and further adhered to a TFT liquid crystaldisplay and placed on a dot printing type backlight. In this sample,polarizing plates (manufactured by NITTO DENKO CO., LTD. with a tradename of SEG1425DU) are singly used on the front and back side,respectively, of a liquid crystal cell without using a viewing anglecompensating film in the TFT liquid crystal display. Ordinary TN cellwas used in the interior of the cell. Any of prism sheets and otherswere not used.

A mat PET diffusing reflective plate was arranged at the down surface ofthe backlight. The obtained collimate system condenses light to thefront face in a similar way to that of a prism light collective sheetand furthermore, transmits circularly polarized light, and a thicknessthereof was extremely thin and takes a value of the order ofone-twentieth of a thickness of 500 μm of a product of two prismsheets+a reflection polarizer combined. A light condensingcharacteristic was on the order ±50° from the vertical direction of thescreen image.

Example 5

A sample was prepared in a similar way to that in Example 4 with theexception that used as a C plate was a retardation plate with aretardation value of 120 nm as measured in a state where being obliquelyinclined by 30°, and a light diffusing pressure sensitive adhesive layerhaving a haze of 92% (of a thickness of 25 μm) obtained by dispersingsilica true spherical particles (with a particle diameter of 4 μm and amixing amount of 30% by weight) into an acrylic pressure sensitiveadhesive (with a thickness of 30 μm and a refractive index of 1.47) isplaced and adhered between a polarizing plate on the front surface sideand a liquid crystal cell of a liquid crystal display. In the sample, alight condensing characteristic in the front face direction was narrowedsubstantially to the order of ±30°. An obtained wide viewing angleliquid crystal display does not cause gray scale inversion within ±60°and maintains a good display characteristic in a viewing anglecharacteristic recognition using a gray scale representation.

Comparative Example 1

Prepared was a methyl ethyl ketone solution (with a solid matter contentof 30 wt %) of a mixture composed of 96 parts by weight of LC242manufactured by BASF Corp. as a polymerizable mesogen compound (a) and 4parts by weight of LC756 manufactured by BASF Corp. as a polymerizablechiral agent (b). A cholesteric liquid crystal film was obtained in asimilar way to that in Example 1 with the exception that the abovedescribed solution was employed. A reflectance spectrum of a cholestericliquid crystal film (a circularly polarizing plate) is shown in FIG. 3.The obtained circularly polarizing plate had a good circularly polarizedlight separating characteristic in the wavelength range of from 650 to750 nm. A pitch length in the obtained cholesteric liquid crystal layerwas 0.44 μm in the vicinity of an ultraviolet illuminated surface (inthe lower layer at 1 μm below the ultraviolet illuminated surface),while being 0.46 μm in the vicinity of the opposite surface (in thelower layer at 1 μm below the opposite surface). It is found from FIG. 3that a reflection band is narrower as compared with that in Example 1.

Comparative Example 2

Prepared was a methyl ethyl ketone solution (with a solid matter contentof 20 wt %) of a mixture composed of 96 parts by weight of the abovedescribed compound (1) as a polymerizable mesogen compound (a), 4 partsby weight of LC756 manufactured by BASF Corp. as a polymerizable chiralagent (b) and 5 parts by weight of IRGACURE 369 (manufactured by ChibaSpecialty Chemicals Corp.) as a photopolymerization initiator (d). Acholesteric liquid crystal film was obtained in a similar way to that inExample 2 with the exception that the above described solution wasemployed. A reflectance spectrum of the cholesteric liquid crystal film(a circularly polarizing plate) is shown in FIG. 4. The obtainedcircularly polarizing plate had a good circularly polarized lightseparating characteristic in the wavelength range of from 600 to 750 nm.A pitch length in the obtained cholesteric liquid crystal layer was 0.4μm in the vicinity of an ultraviolet illuminated surface (in the lowerlayer at 1 μm below the ultraviolet illuminated surface), while being0.45 μm in the vicinity of the opposite surface (in the lower layer at 1μm below the opposite surface). A broad band cholesteric liquid crystalfilm covering visible light was able to be manufactured as a singlelayer in this way. It is found from FIG. 4 that a reflection band isnarrower as compared with that in Example 2.

Comparative Example 3

An aligned film made of polyvinyl alcohol with a thickness of 0.1 μm wasformed on a triacetyl cellulose film and subjected to a rubbingtreatment, and thereafter, three layers made of a cholesteric liquidcrystal polymer with respective selective reflection central wavelengthsof 610 nm, 550 nm and 450 nm and all with a thickness of 2 μm weresequentially formed and aligned. A λ/4 plate obtained by biaxiallystretching a polycarbonate resin film (with a thickness of 80 μm) wasadhered onto the cholesteric liquid crystal film to obtain a linearlypolarizer. A polarizing plate (manufactured by NITTO DENKO CO., LTD.with a trade name of TEG1465DU) was adhered to the linearly polarizer sothat the transmission axis direction coincides with that of thepolarizing plate to obtain a polarizing plate integrated polarizingelement. The polarizing element is used as a lower plate of a TET-LCDand placed on a side light type backlight to measure a brightnessenhancement percentage. A brightness was lowered by 30% or more ascompared with that of Example 1.

INDUSTRIALLY APPLICABILITY

A broad band cholesteric liquid crystal film of the present invention isuseful as a circularly polarizing plate (a reflection polarizer). Thecircularly polarizing plate can be used as a linearly polarizer, aluminaire, a liquid crystal display and others, and in addition thereto,in a polarizing element system and a wide viewing angle liquid crystaldisplay.

1. A broad band cholesteric liquid crystal film comprising: acholesteric liquid crystal film having a reflection bandwidth of 400 nmor more and a pitch length that changes continuously, obtained bycoating a liquid crystal mixture containing a polymerizable mesogencompound (a), a polymerizable chiral agent (b) and a photoisomerizablematerial (c) on a substrate and polymerizing by ultraviolet radiation,wherein the photoisomerizable material (c) is at least onephotoisomerizable material selected from the group consisting ofstilbene of structural formula (I) and azobenzene of structural formula(II)


2. The broad band cholesteric liquid crystal film according to claim 1,wherein the liquid crystal mixture further comprises aphotopolymerization initiator (d).
 3. The broad band cholesteric liquidcrystal film according to claim 1, wherein the polymerizable mesogencompound (a) has one, or two or more of polymerizable functional groups,the polymerizable chiral agent (b) has one, or two or more polymerizablefunctional groups.
 4. A circularly polarizing plate comprising the broadband cholesteric liquid crystal film according to claim
 1. 5. A linerpolarizer comprising the circularly polarizing plate according to claim4 and a λ/4 plate laminated on the circularly polarizing plate.
 6. Thelinear polarizer according to claim 5, wherein the circularly polarizingplate, which comprises the broad band cholesteric liquid crystal film,is laminated on the λ/4 plate so that a pitch length in the film isnarrowed toward the λ/4 plate continuously.
 7. A linear polarizercomprising an absorption polarizer adhered to the linear polarizeraccording to claim 5 so that a transmission axis direction of theabsorption polarizer and a transmission axis of the linearly polarizerare arranged in parallel with each other.
 8. A luminaire comprising thelinear polarizer according to claim 5 on a from surface side of asurface light source having a reflective layer on the back surface sidethereof.
 9. A luminaire comprising the circularly polarizing plateaccording to claim 4 on a front surface side of a surface light sourcehaving a reflective layer on the back surface side thereof.
 10. A liquidcrystal display comprising a liquid crystal cell in a light emittingside of the luminaire according to claim
 9. 11. A polarizing elementsystem comprising: a retardation layer (b) having a front faceretardation (in the normal direction) of almost zero and a retardationof λ/8 or more relative to incident light incoming at an angle of 30° ormore inclined from the normal direction arranged between at least tworeflection polarizer layers having respective selective reflectionwavelength bands of polarized light superimposed on each other, whereineach of the reflection polarizers is the circularly polarizing plateaccording to claim
 4. 12. The polarizing element system according toclaim 11, wherein a selective reflection wavelength of the at least twolayers of the reflection polarizer (a) are superimposed on each other inthe wavelength range 550 nm±10 nm.
 13. The polarizing element systemaccording to claim 11, wherein the retardation layer (b) is a layercomprising a cholesteric liquid crystal phase having a selectivereflection wavelength band other than the visible light region fixed inplanar alignment.
 14. The polarizing element system according to claim11, wherein the retardation layer (b) is a layer comprising a rod-likeliquid crystal fixed in homeotropic alignment state.
 15. The polarizingelement system according to claim 11, wherein the retardation layer (b)is a layer comprising a discotic liquid crystal fixed in nematic phaseor columnar phase alignment state.
 16. The polarizing element systemaccording to claim 11, wherein the retardation layer (b) is a layercomprising a biaxially oriented polymer film.
 17. The polarizing elementsystem according to claim 11, wherein the retardation layer (b) is alayer comprising an inorganic layered compound with negative uniaxialityfixed in alignment state where an optical axis thereof is a normaldirection of a surface thereof.
 18. A wide viewing angle liquid crystaldisplay comprising at least: a backlight system containing a polarizingelement system according to claim 11 to collimate a light from a diffuselight source; a liquid cell transmitting collimated light; a polarizingplate arranged on both sides of the liquid crystal cell; and a viewingangle magnification film, which diffusing transmitted light, arranged ona viewer side of the liquid cell.
 19. The wide viewing angle liquidcrystal display according to claim 18, wherein a λ/4 plate is arrangedon the viewer side (the liquid crystal cell side) of the polarizingelement system so that an axial direction of linearly polarized lighttransmitted and a transmission axis direction of a polarizing plate onthe lower side (the light source side) of the liquid crystal display arearranged in parallel with each other.
 20. The wide viewing angle liquidcrystal display according to claim 19, wherein the viewing anglemagnification film is a diffuse plate substantially having neitherbackscattering nor polarization cancellation.
 21. The wide viewing angleliquid crystal display according to claim 18, wherein the viewing anglemagnification film is a diffuse plate substantially having neitherbackscattering nor polarization cancellation.
 22. The wide viewing angleliquid crystal display according to claim 18, wherein an each layer islaminated with a translucent adhesive or a pressure sensitive adhesive.23. The broad band cholesteric liquid crystal film according to claim 1,wherein the proportion of the polymerizable chiral agent (b) is in therange of from 1 to 20 parts by weight relative to 100 parts by weight ofa total amount of a polymerizable mesogen compound (a) and thepolymerizable chiral agent (b).
 24. A manufacturing method comprisingthe steps of: coating a liquid crystal mixture containing apolymerizable mesogen compound (a), a polymerizable chiral agent (b) anda photoisomerizable material (c) on a substrate polymerizing byultraviolet radiation, and forming a cholesteric liquid crystal filmhaving a reflection bandwidth of 400 nm or more and a pitch length thatchanges continuously, wherein the photopolymerizable material (c) is atleast one photoisomerizable material selected from the group consistingof stilbene of structural formula (I) and azobenzene of structuralformula (II).